WAVES AND VIBRATIONS IN INHOMOGENEOUS STRUCTURES ...
WAVES AND VIBRATIONS IN INHOMOGENEOUS STRUCTURES ...
WAVES AND VIBRATIONS IN INHOMOGENEOUS STRUCTURES ...
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fabrication method. Furthermore, fabrication tolerances are currently pushed to their limits to<br />
obtain acceptable structures, leaving only little room for improvement.<br />
Fabrication of nanophotonic structures using nanoimprint lithography (NIL) [13] is<br />
emerging as a cost-efficient alternative capable of nanometer-to-micrometer-scale pattern<br />
definition in a parallel process. The resolution of NIL is currently limited by the resolution of<br />
the stamp, where e.g. 5-nm linewidth and 14-nm pitch line gratings have been demonstrated<br />
[14].<br />
Here, we demonstrate the feasibility of NIL for the fabrication of SOI-based<br />
nanophotonic components. In our fabrication process lateral resolution better than 30 nm is<br />
achieved on the NIL stamp by high resolution EBL in a thin film of negative resist and<br />
subsequent reactive ion etching (RIE). The pattern is imprinted in a thin film of NIL resist<br />
with a high etch resistance to silicon RIE, which facilitates device definition with the required<br />
high lateral resolution in combination with deep etching into the SOI substrate.<br />
In our device demonstrations, we have realized planar W1 PhCWs, i.e. where the defect<br />
is formed by removing one row of holes in the Γ-K direction of the crystal lattice as shown in<br />
Fig. 1(left). Furthermore, we demonstrate topology-optimized photonic structures [15,16], see<br />
Fig. 2. This type of structure is particular challenging to fabricate with NIL as the frequency<br />
response of the device is highly sensitive to the complicated non-circular features of the<br />
optimized structures and impose local variations in the pattern density. The pattern replication<br />
fidelity is assessed by comparing the measured frequency response with 3D finite-difference<br />
time-domain (FDTD) calculations [17].<br />
2. Nanoimprint lithography fabrication<br />
The fabrication of SOI-based nanophotonic devices is based on thermal NIL [13]. The desired<br />
pattern is defined as a surface relief on the stamp (a silicon wafer) by EBL and RIE. The<br />
pattern is transferred to a thin film of thermoplastic resist on the SOI device wafer by<br />
mechanical deformation as the stamp is embossed into the heated resist. Finally, the pattern is<br />
transferred into the top silicon layer of the SOI wafer by RIE. High resolution and high aspect<br />
ratio of the transferred pattern is obtained by exploiting a high-resolution negative EBL resist<br />
for silicon stamp fabrication in combination with NIL in the thermoplastic resist with high<br />
etching resistance.<br />
The silicon stamp is fabricated by 100 kV EBL (JEOL JBX9300FS) in a 50 nm thin film<br />
of TEBN-1 [18] on a silicon substrate (100 mm diameter and 0.5 mm thick) at an exposure<br />
dose of 9 mC/cm 2 [19]. The written structures are developed in methyl isobutyl ketone<br />
(MIBK) for 20 seconds, rinsed in isopropyl alcohol (IPA), and subsequently transferred 100<br />
nm into the silicon substrate by a highly anisotropic RIE [20]. After etching the silicon, any<br />
remaining resist is removed in oxygen plasma prior to deposition of an anti-sticking layer<br />
formed from a C4F8 plasma and imprinting.<br />
The passivation layer deposition capability of a deep reactive ion etching tool is used to<br />
plasma deposit an anti-sticking layer on the stamp, as originally suggested by Ayón et al [21].<br />
A few monolayers of PTFE-like fluorocarbon polymer is deposited from C4F8 precursor gas,<br />
which is dissociated by plasma to form ions and radical species [22]. The dissociated species<br />
subsequently polymerize on the surface and form a layer of polymerized nCF2. The thickness<br />
of such a fluorocarbon film has been measured to around 5 nm [22]. Without the anti-sticking<br />
layer, the polymer will stick to the stamp, and parts of the polymer pattern are peeled off the<br />
substrate when the stamp and substrate are separated.<br />
The nanophotonic devices are fabricated in a SOI wafer from Soitec (100 mm diameter<br />
and 340 nm silicon on top of 1 μm buried oxide). An 80 nm thin film of mr-I T85 (4 wt%)<br />
[23] is spin coated onto the SOI substrate at a spin speed of 3000 rpm and baked at 150°C for<br />
5 min on a hotplate. The stamp is imprinted in the mr-I T85 film using a pressure of 13 bar for<br />
10 minutes in a parallel plate imprint tool (EVG 520HE) under vacuum (0.01 mbar) and at a<br />
temperature of 140°C.. The stamp and the SOI wafer are separated at a lowered temperature<br />
of 60°C. The imprint parameters result in a complete filling situation of the stamp in the<br />
photonic-crystal structured areas, resulting in 80 nm deep holes in the mr-I T85 resist. The<br />
#76773 - $15.00 USD Received 6 November 2006; revised 18 January 2007; accepted 19 January 2007<br />
(C) 2007 OSA 5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1263